Obsessive Compulsive Disorder (OCD) is now recognized as a common mental disorder, occurring in perhaps three million people in this country alone. In adolescents, the prevalence maybe as high as one percent. Follow-up studies in our Branch show a chronic course for the disorder with increasing severity of depression and anxiety over time. Clomipramine has a highly specific beneficial effect on obsessions while Desmethylimipramine (DMI) another tricyclic antidepressant did not differ from placebo. New studies with brain imaging (PET and CT scan), as well as a survey of patients with Sydenham's chorea, implicate abnormalities in the basal ganglia while spinal fluid studies show that low concentration of CSF 5-HIAA, the major serotonin metabolite, is associated with more severe OCD.